mbcache2: Use referenced bit instead of LRU

Currently we maintain perfect LRU list by moving entry to the tail of
the list when it gets used. However these operations on cache-global
list are relatively expensive.

In this patch we switch to lazy updates of LRU list. Whenever entry gets
used, we set a referenced bit in it. When reclaiming entries, we give
referenced entries another round in the LRU. Since the list is not a
real LRU anymore, rename it to just 'list'.

In my testing this logic gives about 30% boost to workloads with mostly
unique xattr blocks (e.g. xattr-bench with 10 files and 10000 unique
xattr values).

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 7 insertions, 4 deletions

diff --git a/include/linux/mbcache2.h b/include/linux/mbcache2.h
index b6f160ff2533..c934843a6a31 100644
--- a/include/linux/mbcache2.h
+++ b/include/linux/mbcache2.h
@@ -10,8 +10,8 @@
 struct mb2_cache;
 
 struct mb2_cache_entry {
-	/* LRU list - protected by cache->c_lru_list_lock */
-	struct list_head	e_lru_list;
+	/* List of entries in cache - protected by cache->c_list_lock */
+	struct list_head	e_list;
 	/* Hash table list - protected by bitlock in e_hash_list_head */
 	struct hlist_bl_node	e_hash_list;
 	atomic_t		e_refcnt;
@@ -19,8 +19,11 @@ struct mb2_cache_entry {
 	u32			e_key;
 	/* Block number of hashed block - stable during lifetime of the entry */
 	sector_t		e_block;
-	/* Head of hash list (for list bit lock) - stable */
-	struct hlist_bl_head	*e_hash_list_head;
+	/*
+	 * Head of hash list (for list bit lock) - stable. Combined with
+	 * referenced bit of entry
+	 */
+	unsigned long		_e_hash_list_head;
 };
 
 struct mb2_cache *mb2_cache_create(int bucket_bits);